High temperature furnaces are customarily heated by at least one and usually by a multiplicity of burners supported externally and removably on one or more walls of the furnace chamber and serve to heat this chamber to temperatures ranging between 1,600.degree. and 3,000.degree. F. The wall orifices into which the burner nozzles project are subjected to extremely hostile and harsh conditions. Heretofore it has been substantially the universal practice to utilise a cast burner block or collar replaceably mounted in the furnace wall and surrounding the burner nozzle. Such blocks, commonly known as "castables", are molded to shape and size from high temperature cement compositions. Such blocks are not only extremely heavy to install, service and replace but have very poor thermal shock absorbing capabilities. In consequence, they are subject to spalling, chipping and cracking, particularly during furnace start-up and shut-down operations. Larger embodiments of these conventional castable blocks weigh 800 to 1,100 lbs each and, in some furnace installations, such blocks may have to be installed 50 to 80 feet above ground level. This requires special equipment and poses hazards to the furnace, the equipment and particularly to the crews performing the service and installation operations.